Effect of acute global ischemia on the upper limit of vulnerability: a simulation study

The goal of this modeling research is to provide mechanistic insight into the effect of altered membrane kinetics associated with 5-12 min of acute global ischemia on the upper limit of cardiac vulnerability (ULV) to electric shocks. We simulate electrical activity in a finite-element bidomain model of a 4-mm-thick slice through the canine ventricles that incorporates realistic geometry and fiber architecture. Global acute ischemia is represented by changes in membrane dynamics due to hyperkalemia, acidosis, and hypoxia. Two stages of acute ischemia are simulated corresponding to 5-7 min (stage 1) and 10-12 min (stage 2) after the onset of ischemia. Monophasic shocks are delivered in normoxia and ischemia over a range of coupling intervals, and their outcomes are examined to determine the highest shock strength that resulted in induction of reentrant arrhythmia. Our results demonstrate that acute ischemia stage 1 results in ULV reduction to 0.8A from its normoxic value of 1.4A. In contrast, no arrhythmia is induced regardless of shock strength in acute ischemia stage 2. An investigation of mechanisms underlying this behavior revealed that decreased postshock refractoriness resulting mainly from 1) ischemic electrophysiological substrate and 2) decrease in the extent of areas positively-polarized by the shock is responsible for the change in ULV during stage 1. In contrast, conduction failure is the main cause for the lack of vulnerability in acute ischemia stage 2. The insight provided by this study furthers our understanding of mechanisms by which acute ischemia-induced changes at the ionic level modulate cardiac vulnerability to electric shocks.     

Studies on re-entrant arrhythmias and ectopic beats in excitable tissues by bifurcation analyses.

A phase-plane bifurcation analysis is a useful way to theoretically understand how various types of arrhythmias may arise from excitable tissues. In this paper, we have performed phase-plane bifurcation analysis to characterize arrhythmogenic states in excitable tissues. To achieve this, we have first formulated a model which is simple enough to be mathematically tractable, yet captures the non-linear features of cardiac excitation and conduction. In this model, single cells are connected in a circular fashion by gap conductances. Each cell carries the following two types of currents: a passive outward current and an inward "excitable" current which contains an activation and an inactivation gate. The activation gate is responsible for the upstroke of action potential and inactivation gate is responsible for the termination of the plateau potential. With this model, we have constructed bifurcation diagrams as a function of a bifurcation parameter. The parameter chosen as the bifurcation parameter has the property of raising maximum diastolic potential while shorting the refractory period. Our analysis revealed the existence of three distinct multi-stable phases in certain ranges of the bifurcation parameter: (1) bistability between a rotor and a quiescent state, (2) bistability between rotor and ectopic beats, and (3) three stable states co-existing among quiescent state, rotor, and ectopic beats. In these three regions, external impulses exert very distinct effects: In region 1, a brief current pulse can annihilate a re-entrant arrhythmia to quiescence. To initiate re-entry from a quiescent tissue, however, it takes two pulses (a primary pulse followed by a premature pulse at a site different from the "primary" site). In region 2, a brief pulse can convert a re-entrant arrhythmia to ectopic beats. To convert the ectopic beats back to circus movement, these beats have to be suppressed by a few brief current pulses to initiate one-way propagation. Depending on the frequency and strength of impulses in region 3, the tissue can switch back and forth among quiescence, circus movement, and ectopic beats. For comparison, we have also included a more complete Beeler-Reuter cardiac cell model in our analysis and obtained essentially the same results. From the behavioral similarities of these models, we conclude that re-entrant and ectopic arrhythmias must be intrinsic properties of excitable tissues and external stimuli can convert one mode of arrhythmia to another in the multistability regions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cardiac excitation mechanisms, wavefront dynamics and strength-interval curves predicted by 3D orthotropic bidomain simulations

The assessment and understanding of cardiac excitation mechanisms is very important for the development and improvement of implantable cardiac devices, pacing protocols, and arrhythmia treatments. Previous bidomain simulation studies have investigated cathodal and anodal make/break mechanisms of cardiac excitation and strength-interval (S-I) curves in two-dimensional sheets or cylindrical domains, that by symmetry reduce to the two-dimensional case. In this work, cathodal and anodal S-I curves are studied by means of detailed bidomain simulations which include: (i) three-dimensional cardiac slabs; (ii) transmural fiber rotation; (iii) unequal orthotropic anisotropy of the conducting media; (iv) incorporation of funny and electroporation currents in the ventricular membrane model. The predicted shape of cathodal and anodal S-I curves exhibit the same features of the S-I curves observed experimentally and the break/make transition coincides with the final descending phase of the S-I curves. Away from the break/make transition, only the break or make excitation mechanism is observed independently of the stimulus strength, whereas within an interval at the break/make transition, new paradoxical excitation behaviors are observed that depend on the stimulus strength.     

Reentrant waves in a ring of embryonic chick ventricular cells imaged with a Ca2+ sensitive dye

According to the classic model initially formulated by Mines, reentrant cardiac arrhythmias may be associated with waves circulating in a ring geometry. This study was designed to study the dynamics of reentry in a ring geometry of cardiac tissue culture. Reentrant calcium waves in rings of cultured embryonic chick cardiac myocytes were imaged using a macroscope to monitor the fluorescence of intracellular Calcium Green-1 dye. The rings displayed a variety of stable rhythms including pacemaker activity and spontaneous reentry. Waves originating from a localized pacemaker could lead to reentry as a consequence of unidirectional block. In addition, more complex patterns were observed due to the interactions between reentrant and pacemaker rhythms. These rhythms included instances in which pacemakers accelerated the reentrant rhythm, and instances in which the excitation was blocked in the vicinity of pacemakers. During reentrant activity an appropriately timed electrical stimulus could induce resetting of activity or cause complete annihilation of the propagating waves. This experimental preparation reveals many spontaneously occuring complex rhythms. These complex rhythms are hypothesized to reflect interactions between spontaneous pacemakers, wave propagation, refractory period, and overdrive suppression. This preparation may serve as a useful model system to further investigate complex dynamics arising during reentrant rhythms in cardiac tissue.     

The role of transmural ventricular heterogeneities in cardiac vulnerability to electric shocks

Transmural electrophysiological heterogeneities have been shown to contribute to arrhythmia induction in the heart; however, their role in defibrillation failure has never been examined. The goal of this study is to investigate how transmural heterogeneities in ionic currents and gap-junctional coupling contribute to arrhythmia generation following defibrillation strength shocks. This study used a 3D anatomically realistic bidomain model of the rabbit ventricles. Transmural heterogeneity in ionic currents and reduced sub-epicardial intercellular coupling were incorporated based on experimental data. The ventricles were paced apically, and truncated-exponential monophasic shocks of varying strength and timing were applied via large external electrodes. Simulations demonstrate that inclusion of transmural heterogeneity in ionic currents results in an increase in vulnerability to shocks, reflected in the increased upper limit of vulnerability, ULV, and the enlarged vulnerable window, VW. These changes in vulnerability stem from increased post-shock dispersion in repolarisation as it increases the likelihood of establishment of re-entrant circuits. In contrast, reduced sub-epicardial coupling results in decrease in both ULV and VW. This decrease is caused by altered virtual electrode polarisation around the region of sub-epicardal uncoupling, and specifically, by the increase in (1) the amount of positively polarised myocardium at shock-end and (2) the spatial extent of post-shock wavefronts.     

In vivo magnetic and electric recordings from nerve bundles and single motor units in mammalian skeletal muscle. Correlations with muscle force

Recent advances in the technology of recording magnetic fields associated with electric current flow in biological tissues have provided a means of examining action currents that is more direct and possibly more accurate than conventional electrical recording. Magnetic recordings are relatively insensitive to muscle movement, and, because the recording probes are not directly connected to the tissue, distortions of the data due to changes in the electrochemical interface between the probes and the tissue are eliminated. In vivo magnetic recordings of action currents of rat common peroneal nerve and extensor digitorum longus (EDL) muscle were obtained by a new magnetic probe and amplifier system that operates within the physiological temperature range. The magnetically recorded waveforms were compared with those obtained simultaneously by conventional, extracellular recording techniques. We used the amplitude of EDL twitch force (an index of stimulus strength) generated in response to graded stimulation of the common peroneal nerve to enable us to compare the amplitudes of magnetically recorded nerve and muscle compound action currents (NCACs and MCACs, respectively) with the amplitudes of electrically recorded nerve compound action potentials (NCAPs). High, positive correlations to stimulus strength were found for NCACs (r = 0.998), MCACs (r = 0.974), and NCAPs (r = 0.998). We also computed the correlations of EDL single motor unit twitch force with magnetically recorded single motor unit compound action currents (SMUCACs) and electrically recorded single motor unit compound action potentials (SMUCAPs) obtained with both a ring electrode and a straight wire serving as a point electrode. Only the SMUCACs had a relatively strong positive correlation (r = 0.768) with EDL twitch force. Correlations for ring and wire electrode-recorded SMUCAPs were 0.565 and -0.366, respectively. This study adds a relatively direct examination of action currents to the characterization of the normal biophysical properties of peripheral nerve, muscle, and muscle single motor units.     

Experimental studies on antiarrhythmic and antiseizure effects of polyunsaturated fatty acids in excitable tissues

It has been shown that in animals, and probably in humans, n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. We discuss our recent studies on the antiarrhythmic actions of PUFAs. PUFAs stabilize the electrical activity of isolated cardiac myocytes by requiring a stronger electrical stimulus to elicit an action potential and by markedly prolonging the refractory period. These electrophysiologic effects are the result of specific modulation of ion currents, particularly of the voltage-dependent sodium current and of the L-type calcium currents across sarcolemmal phospholipid membranes. This appears to be the probable major antiarrhythmic mechanism of PUFAs. However, they also similarly affect neuronal ion channels with potentially important functional effects on the nervous system.     

Pheromone-stimulated locomotory and orientation responses in the American cockroach

Males of Periplaneta americana respond to the sex pheromone secreted by females with increased locomotion and positive chemo-orientation. Both sexes orient toward aggregation pheromone without an increase in locomotion. Immediately following removal of one antenna males exhibit ‘circus’ movements, but after 2 days orient toward pheromones; a bi-modal mechanism of chemo-orientation is proposed. Cockroaches turn away from air currents, but orient toward air currents carrying sex or aggregation pheromone, suggesting the possibility of up-wind orientation. Antennae deflect upward and outward when pheromone is first perceived; the head then moves toward the pheromone source. Following removal of one antenna, the pattern of head and antennal movement changes in a manner which enhances the sweeping of the intact antenna.     

Functional and electrophysiologic effects of polyunsaturated fatty acids on exictable tissues: heart and brain.

It has been shown in animals and probably in humans, that n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. The free PUFAs stabilize the electrical activity of isolated cardiac myocytes by inhibiting sarcolemmal ion channels, so that a stronger electrical stimulus is required to elicit an action potential and the relative refractory period is markedly prolonged. This appears at present to be the probable major antiarrhythmic mechanism of the PUFAs. They similarly inhibit the Na+ and Ca2+ currents in rat hippocampal neurons which results in an increase in the electrical threshold for generalized seizures using the cortical stimulation model in rats.     

The Antiarrhythmic and Anticonvulsant Effects of Dietary N-3 Fatty Acids

It has been shown in animals and probably in humans, that n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. We report recent studies on the antiarrhythmic actions of PUFAs. The PUFAs stabilize the electrical activity of isolated cardiac myocytes by modulating sarcolemmal ion channels, so that a stronger electrical stimulus is required to elicit an action potential and the refractory period is markedly prolonged. Inhibition of voltage-dependent sodium currents, which initiate action potentials in excitable tissues, and of the L-type calcium currents, which initiate release of sarcoplasmic calcium stores that increase cytosolic free calcium concentrations and activate the contractile proteins in myocytes, appear at present to be the probable major antiarrhythmic mechanism of the PUFAs. Received: 27 May 1999/Revised: 20 July 1999     

An experimental analysis of “empathic” response: Effects of pain reactions of pigeon upon other pigeon's operant behavior.

Suppression of operant behavior by exposure to pain reactions of conspecifics was examined with pigeons. Three groups of pigeons were trained on a VI schedule, and were then exposed to the pain reactions of an adjoining bird to electric shocks. Although every subject showed suppression of responding, the suppression decreased with repeated exposures. Following this assessment, a conditioning group received conditioned suppression training in which the pain reaction of the adjoining bird was the CS and an electric shock was the US; a shock exposure group received the electric shock without any explicit CS; and, a no-shock group did not receive any shock. After these treatments, every group was exposed to the pain reactions of the adjoining bird (test 1). The conditioning group and the shock exposure group showed clear suppression in responding, but the no-shock group did not.The no-shock group then received the shock exposure treatment and the conditioned suppression training succesively, and the shock exposure group received the conditioned suppression training. Results of tests with the pain reaction of the adjoining bird supported the results of the test 1, however, suppression caused by the shock exposure was not so clear in the no-shock group.The present results demonstrated that conspecific behavior can become a CS by conditioned suppression training, and, the behavior to an aversive stimulus can acquire aversive properties for other conspecifics when they have shared the exposure to the same aversive stimulus.     

Impulse responses of automaticity in the Purkinje fiber

We examined the effects of brief current pulses on the pacemaker oscillations of the Purkinje fiber using the model of McAllister , Noble, and Tsien (1975. J. Physiol. [Lond.]. 251:1-57). This model was used to construct phase-response curves for brief electric stimuli to find "black holes," where rhythmic activity of the Purkinje fiber ceases. In our computer simulation, a brief current stimulus of the right magnitude and timing annihilated oscillations in membrane potential. The model also revealed a sequence of alternating periodic and chaotic regimes as the strength of a steady bias current is varied. We compared the results of our computer simulations with experimental work on Purkinje fibers and pointed out the importance of modeling results of this kind for understanding cardiac arrhythmias.     

Mechanisms of cardiac cell excitation with premature monophasic and biphasic field stimuli: a model study.

The mechanisms by which extracellular electric field stimuli induce the (re)excitation of cardiac cells in various stages of refractoriness are still not well understood. We modeled the interactions between an isolated cardiac cell and imposed extracellular electric fields to determine the mechanisms by which relatively low-strength uniform monophasic and biphasic field stimuli induce premature reexcitations. An idealized ventricular cell was simulated with 11 subcellular membrane patches, each of which obeyed Luo-Rudy (phase 1) kinetics. Implementing a standard S1-S2 pulse protocol, strength-interval maps of the cellular excitatory responses were generated for rectangular monophasic and symmetric biphasic field stimuli of 2, 5, 10, and 20 ms total duration. In contrast to previously documented current injection studies, our results demonstrate that a cardiac cell exhibits a significantly nonmonotonic excitatory response to premature monophasic and, to a much lesser degree, biphasic field stimuli. Furthermore, for monophasic stimuli at low field strengths, the cell is exquisitely sensitive to the timing of the shock, demonstrating a classic all-or-none depolarizing response. However, at higher field strengths this all-or-none sensitivity reverts to a more gradual transition of excitatory responses with respect to stimulus prematurity. In contrast, biphasic stimuli produce such graded responses at all suprathreshold stimulus strengths. Similar behaviors are demonstrated at all S2 stimulus durations tested. The generation of depolarizing (sodium) currents is triggered by one or more of the sharp field gradient changes produced at the stimulus edges-i.e., make, break, and transphasic (for biphasic stimuli)-with the magnitude of these edge-induced current contributions dependent on both the prematurity and the strength of the applied field. In all cases, however, depolarizing current arises from the partial removal of sodium inactivation from at least part of the cell, because of either the natural process of repolarization or a localized acceleration of this process by the impressed field.     

The mechanisms of the vulnerable window: the role of virtual electrodes and shock polarity

Vulnerability and defibrillation are mechanistically dependent upon shock strength, polarity, and timing. We have recently demonstrated that shock-induced virtual electrode polarization (VEP) may induce reentry. However, it remains unclear how the VEP mechanism may explain the vulnerable window and polarity dependence of vulnerability. We used a potentiometric dye and optical mapping to assess the anterior epicardial electrical activity of Langendorff-perfused rabbit hearts (n = 7) during monophasic shocks (+/-100 V and +/-200 V, duration of 8 ms) applied from a transvenous defibrillation lead at various coupling intervals. Arrhythmias were induced in a coupling interval and shock polarity dependent manner: (i) anodal and cathodal shocks induced arrhythmias in 33.2 +/- 30.1% and 53.1 +/- 39.3% cases (P < 0.01), respectively, and (ii) the vulnerable window was located near the T-wave. Optical maps revealed that VEP was also modulated by the coupling interval and shock polarity. Recovery of excitability produced by negative polarization, known as de-excitation, and the resulting reentry was more readily achieved during the relative refractory period than the absolute refractory period. Furthermore, anodal shocks produced wavefronts propagating in an inward direction with respect to the electrode, whereas cathodal shocks propagated in an outward direction. Wavefronts produced by anodal shocks were more likely to collide and annihilate each other than those caused by cathodal shocks. The probability of degeneration of the VEP-induced phase singularity into a sustained arrhythmia depends upon the gradient of VEP and the direction of the VEP-induced wavefront. The VEP gradient depends upon the coupling interval, while the direction depends upon shock polarity; these factors explain the vulnerable window and polarity-dependence of vulnerability, respectively.     

The role of mechanoelectric feedback in vulnerability to electric shock

Experimental and clinical studies have shown that ventricular dilatation is associated with increased arrhythmogenesis and elevated defibrillation threshold; however, the underlying mechanisms remain poorly understood. The goal of the present study was to test the hypothesis that (1) stretch-activated channel (SAC) recruitment and (2) geometrical deformations in organ shape and fiber architecture lead to increased arrhythmogenesis by electric shocks following acute ventricular dilatation. To elucidate the contribution of these two factors, the study employed, for the first time, a combined electro-mechanical simulation approach. Acute dilatation was simulated in a model of rabbit ventricular mechanics by raising the LV end-diastolic pressure from 0.6 (control) to 4.2 kPa (dilated). The output of the mechanics model was used in the electrophysiological model. Vulnerability to shocks was examined in the control, the dilated ventricles, and in the dilated ventricles that also incorporated currents through SAC as a function of local strain, by constructing vulnerability grids. Results showed that dilatation-induced deformation alone decreased upper limit of vulnerability (ULV) slightly and did not result in increased vulnerability. With SAC recruitment in the dilated ventricles, the number of shock-induced arrhythmia episodes increased by 37% (from 41 to 56) and the lower limit of vulnerability (LLV) decreased from 9 to 7 V/cm, while ULV did not change. The heterogeneous activation of SAC caused by the heterogeneous fiber strain in the ventricular walls was the main reason for increased vulnerability to electric shocks since it caused dispersion of electrophysiological properties in the tissue, resulting in postshock unidirectional block and establishment of reentry.     

Influence of stimulus intensity on the soleus H-reflex amplitude and modulation during locomotion

Diverging results have been reported regarding the modulation and amplitude of the soleus H-reflex measured during human walking and running. A possible explanation to this could be the use of too high stimulus strength in some studies while not in others. During activities like walking and running it is necessary to use a small M-wave to control the effective stimulus strength during all phases of the movement. This implies that the descending part of the H-reflex recruitment curve is being used, which may lead to an unwanted suppression of the H-reflex due to limitations imbedded within the H-reflex methodology itself.Accordingly, the purpose of the present study was to study the effect on the soleus H-reflex during walking and running using stimulus intensities normally considered too high (up to 45% Mmax).Using M-waves of 25–45% Mmax as opposed to 5–25% Mmax showed a significant suppression of the peak H-reflex during the stance phase of walking, while no changes were observed during running. No differences were observed regarding modulation pattern. So a possible use of too high stimulus intensity cannot explain the differences mentioned. The surprising result in running may be explained by the much higher voluntary muscle activity, which implies the existence of a V-wave influencing the H-reflex amplitude in positive direction.     

Cardiac arrhythmias modelled by Cai-inactivated Ca2+ channels

A model of cardiac cells incorporating the membrane potential and the intracellular calcium concentration as the two dynamical variables is developed. This model is applied to simple systems of cells to investigate its behavior as a function of the model parameters. Rational entrainment is observed in systems of two coupled pacemaker cells. The propagation of the membrane potential and intracellular calcium concentration through a sheet is simulated. Behavior suggestive of circus movement tachycardias is observed.     

Different sensitivities of two mechanisms of excitation waves in heart tissue to anti-arrhythmia agents--blockers of fast sodium currents

In experiments with rabbit ventricular tissue sensitivity of two kinds of spiral wave sources of excitation to fast sodium current inhibition was compared. These spiral wave sources were circulation in a ring around an obstacle and circulation in tissue without an obstacle (reverberator). It was observed that after application of antiarrhythmic drugs lidocaine or mexiletine there was a prominent growth of cycle length of reverberator during first few seconds of circulation and a little change of cycle length for the circus movement around obstacle. It is proposed that different sensitivity of both kinds of spiral wave sources to antiarrhythmic drugs can be used for their distinguishing in clinical practice.     

Effect of rj1rj1 (non-nodulating) soybeans on nodulation of near isogenic Rj1Rj1 plants in nutrient culture

An earlier proposal (Can. J. Microbiol. 7: 851; 1961) that rj1rj1 (non-nodulating) soybeans (Glycine max (L.) Merr.) excrete a substance that inhibits nodulation of Rj1 Rj1 (nodulating) plants was tested. Using near isogenic lines (isolines) of "Clark" and "Harosoy" soybeans, we consistently found nonsignificant reduction in nodule number and acetylene reduction per Rj1Rj1 plant grown in association with their rj1rj1 counterparts: these results suggest that a nodulation inhibitor is not associated with the rj1 gene. Reducing the number of plants grown in each pot produced significant (P = 0.05) reductions in nodule number per Rj1Rj1 plant, and resembled the observations of the earlier report. On this basis, we suggest that the reported inhibition of nodulation was due to a failure to detoxify or remove an inhibitor (possibly nitrate) already present in the nutrient solution. Both Clark isolines removed nitrate from their nutrient solutions at similar rates. Harosoy rj1rj1 plants removed nitrate at a significantly (P - 0.05) slower rate than Harosoy Rj1Rj1 plants, but the differences were not correlated (P = 0.05) with the small observed decreases in nodulation. These differences in nitrate uptake were highly correlated (P = 0.01) with reduced dry weight per Harosoy rj1rj1 plant.